Chemical etching of natural fluorapatite crystals in acid solutions studied with the scanning electron microscope

Abstract

Different aspects of the chemical etching process for natural fluorapatite (FAP) crystals in pure phosphoric acid solutions and for those with additives of Ca2+ and H2SO4 have been studied by means of the scanning electron microscope. The resulting experimental data obtained in the etching process are very sensitive to dissolving conditions, type of crystal face, and the presence or absence of any epitaxial coatings on the surface of dissolving crystals. It was found that the pyramidal crystal faces of the FAP have the highest dissolution rate value and, as a result, are not etched at all. The prism and pinacoid faces usually are covered by hexagonal etch pits under the same conditions. Additives of Ca2+ to the phosphoric acid solution result in transition from a chemical etching to a chemical polishing process, and additives of H2SO4 result in epitaxial coatings of CaSO4· xH2O (x = 0,0.5, 2) and shapeless pits formation. Copyright © 1995 Wiley Periodicals, Inc.